Nick, this may explain why you are not 100% happy with the top end of the cartridge. The plot shows a standard EMT MC into 100 Ohms (cap not specified). It shows +4db lift at 20Khz which will certainly be audible. Naturally, the Brinkman version will be different in some ways but if it is based on the same basic design it may have similar characteristics. How do Brinkman recommend you load it.

(PS: don't have heart failure at the square wave ringing as this is cutter ringing - not a fault with the cartridge)